Magnetoresistive random access memory (MRAM) is a non-volatile computer memory type that stores data with magnetic storage elements. The magnetic storage elements (or cells) usually are formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a permanent magnet set to a particular polarity. The other plate's field changes to match that of an external field.
A magnetic memory device typically comprises an array of such memory cells, each cell being individually addressable by a particular word line and bit line arranged at right angles, above and below the cell, respectively. When current is passed through them, an induced magnetic field is created at the junction, which the writable plate picks up, in order to write data to the memory cell. It should be noted that as magnetic memory cells are scaled down in size, there comes a time when the induced field used to write data to a particular cell overlaps adjacent cells over a small area, leading to potential false writes.
Reading of the memory cells is accomplished by measuring the electrical resistance of the cell. A particular cell typically is selected by powering an associated transistor, which switches current from a supply line through the cell to ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes due to the orientation of the fields in the two plates. By measuring the resulting current, the resistance inside any particular cell can be determined, and from this the polarity of the writable plate.
It is known to use bilayer in MRAM storage elements that comprises (i) a magnetically hard layer and (ii) a switching material, like FeRh, that exhibits a transition from antiferromagnetic to ferromagnetic at a transition temperature less than the Curie temperature of the magnetically hard layer to assist in the control of switching the memory cell. Published U.S. patent application Pub. No. 2005/0281081 A1 describes one such memory cell.